Use string values when checking enum TypedDict keys

[snoopuppy582]

Fixes #21722.

TypedDict indexing treats an Enum literal's stored member name as a string key. This accepts plain Enum members that raise KeyError at runtime. For string enums, it can also check the wrong field: name = "value" indexes "value", not "name".

Require a string subtype, then resolve string-enum values through the existing Enum attribute plugin before checking the key. Nested enum values are resolved the same way. Using the Enum descriptor avoids following an overridden .value property; custom constructors with unknown string values and recursive stub declarations are rejected as nonliteral keys.

The regressions cover ordinary Enum reads and writes, both string-enum forms, mismatched names and values, literal unions, nested values, overridden properties and unsupported constructors. The four new cases fail on the previous PR head and pass with this change.

Validation on Windows, Python 3.13.12:

• TypedDict and Enum data suites: 496 passed.
• Mypy self-check: 196 source files passed.
• Changed-file pre-commit hooks and git diff --check: passed.
• A check against the real typeshed agrees with five runtime lookups and rejects both KeyError controls.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅